Cone-beam computed tomography and its applications in dental and maxillofacial radiology.

Cone-beam computed tomography (CBCT) was first used in dental and maxillofacial radiology (DMFR) at the end of the 1990s. Since then, it has been successfully established as the standard three-dimensional radiographic imaging technique in DMFR, with a wide variety of applications in this field. This manuscript briefly reviews the background information on the technology and summarises available data on effective dose and dose optimisation. In addition, typical clinical applications and indications of the technique in DMFR are presented.

Correlation of objective image quality and working length measurements in different CBCT machines: An ex vivo study.

To investigate potential correlations between objective CBCT image parameters and accuracy in endodontic working length determination ex vivo. Contrast-to-noise ratio (CNR) and spatial resolution (SR) as fundamental objective image parameters were examined using specific phantoms in seven different CBCT machines. Seven experienced observers were instructed and calibrated. The order of the CBCTs was randomized for each observer and observation. To assess intra-operator reproducibility, the procedure was repeated within six weeks with a randomized order of CBCT images. Multivariate analysis (MANOVA) did not reveal any influence of the combined image quality factors CNR and SR on measurement accuracy. Inter-operator reproducibility as assessed between the two observations was poor, with a mean intra-class correlation (ICC) of 0.48 (95%-CI  0.38, 0.59) for observation No. 1. and 0.40 (95%-CI 0.30, 0.51) for observation No. 2. Intra-operator reproducibility pooled over all observers between both observations was only moderate, with a mean ICC of 0.58 (95%-CI 0.52 to 0.64). Within the limitations of the study, objective image quality measures and exposure parameters seem not to have a significant influence on accuracy in determining endodontic root canal lengths in CBCT scans. The main factor of variance is the observer.

The effect of wavelet and discrete cosine transform compression of digital radiographs on the detection of subtle proximal caries. ROC analysis.

The study compared diagnostic performances of 2 different image compression methods: JPEG (discrete cosine transform; Joint Photographic Experts Group compression standard) versus JPEG2000 (discrete wavelet transform), both at a compression ratio of 12:1, from the original uncompressed TIFF radiograph with respect to the detection of non-cavitated carious lesions. Therefore, 100 approximal surfaces of 50 tooth pairs were evaluated on the radiographs by 10 experienced observers using a 5-point confidence scale. Observations were carried out on a standardized viewing monitor under subdued light conditions. The proportion of diseased surfaces was balanced to approximately 50% to avoid bias. True caries status was assessed by serial ground sectioning and microscopic evaluation. A non-parametric receiver operating characteristic analysis revealed non-significant differences between the 3 image modalities, as computed from the critical ratios z not exceeding +/-2 (JPEG/JPEG2000, z = -0.0339; TIFF/JPEG2000, z = 0.251;TIFF/JPEG, z = 0.914). The mean area beneath the curve was highest for TIFF (0.604) followed by JPEG2000 (0.593) and JPEG (0.591). Both intra-rater and inter-rater agreement were significantly higher for TIFF (kappa(intra) = 0.52; kappa(inter) = 0.40) and JPEG2000 images (kappa(intra) = 0.49; kappa(inter) = 0.38) than for JPEG images (kappa(intra) = 0.33; kappa(inter) = 0.35). Our results indicate that image compression with typical compression algorithms at rates yielding storage sizes of around 50 kB is sufficient even for the challenging task of radiographic detection of non-cavitated carious approximal lesions.

Spatial resolution in CBCT machines for dental/maxillofacial applications-what do we know today?

Spatial resolution is one of the most important parameters objectively defining image quality, particularly in dental imaging, where fine details often have to be depicted. Here, we review the current status on assessment parameters for spatial resolution and on published data regarding spatial resolution in CBCT images. The current concepts of visual [line-pair (lp) measurements] and automated [modulation transfer function (MTF)] assessment of spatial resolution in CBCT images are summarized and reviewed. Published measurement data on spatial resolution in CBCT are evaluated and analysed. Effective (i.e. actual) spatial resolution available in CBCT images is being influenced by the two-dimensional detector, the three-dimensional reconstruction process, patient movement during the scan and various other parameters. In the literature, the values range between 0.6 and 2.8 lp mm(-1) (visual assessment; median, 1.7 lp mm(-1)) vs MTF (range, 0.5-2.3 cycles per mm; median, 2.1 lp mm(-1)). Spatial resolution of CBCT images is approximately one order of magnitude lower than that of intraoral radiographs. Considering movement, scatter effects and other influences in real-world scans of living patients, a realistic spatial resolution of just above 1 lp mm(-1) could be expected.

Pose determination of a cylindrical (dental) implant in three-dimensions from a single two-dimensional radiograph.

OBJECTIVES: The aim was to develop an analytical algorithm capable of determining localization and orientation of a cylindrical (dental) implant in three-dimensional (3D) space from a single radiographic projection. METHODS: An algorithm based on analytical geometry is introduced, exploiting the geometrical information inherent in the 2D radiographic shadow of an opaque cylindrical implant (RCC) and recovering the 3D co-ordinates of the RCC's main axis within a 3D Cartesian co-ordinate system. Prerequisites for the method are a known source-to-receptor distance at a known locus within the flat image receptor. RESULTS: Accuracy, assessed from a small feasibility experiment in atypical dental radiographic geometry, revealed mean absolute errors for the critical depth co-ordinate ranging between 0.5 mm and 5.39 mm. This translates to a relative depth error ranging from 0.19% to 2.12%. CONCLUSIONS: Experimental results indicate that the method introduced is capable of providing geometrical information important for a variety of applications. Accuracy has to be enhanced by means of automated image analysis and processing methods.

Software for automated application of a reference-based method for a posteriori determination of the effective radiographic imaging geometry.

OBJECTIVES: Presentation and validation of software developed for automated and accurate application of a reference-based algorithm (reference sphere method: RSM) inferring the effective imaging geometry from quantitative radiographic image analysis. METHODS: The software uses modern pattern recognition and computer vision algorithms adapted for the particular application of automated detection of the reference sphere shadows (ellipses) with subpixel accuracy. It applies the RSM algorithm to the shadows detected, thereby providing three-dimensional Cartesian coordinates of the spheres. If the three sphere centres do not lie on one line, they uniquely determine the imaging geometry. Accuracy of the computed coordinates is investigated in a set of 28 charge-coupled device (CCD)-based radiographs of two human mandible segments produced on an optical bench. Each specimen contained three reference spheres (two different radii r1=1.5 mm, r2=2.5 mm). True sphere coordinates were assessed with a manually operated calliper. Software accuracy was investigated for a weighted and unweighted algebraic ellipse-fitting algorithm. RESULTS: The critical depth- (z-) coordinates revealed mean absolute errors ranging between 1.1+/-0.7 mm (unweighted version; r=2.5 mm) and 1.4+/-1.4 mm (weighted version, r=2.5 mm), corresponding to mean relative errors between 5% and 6%. Outliers resulted from complete circular dense structure superimposition and one obviously deformed reference sphere. CONCLUSIONS: The software provides information fundamentally important for the image formation and geometric image registration, which is a crucial step for three-dimensional reconstruction from > or =2 two-dimensional views.

[Root growth after wisdom tooth transposition. Case documentation with proposal for a radiologic method to quantify growth]. / Wurzelwachstum nach Weisheitszahntransposition. Falldokumentation mit Vorschlag einer radiologischen Methode zur Wachstumsquantifizierung.

OBJECTIVES: To introduce a method for assessment of root growth on panoramic radiographs exemplified by a case report on an autotransplanted lower third molar. MATERIAL AND METHODS: A lower left third molar (T) with incomplete root formation was transplanted for replacement of the lower left first molar in an 18-year-old woman. T was monitored clinically and, together with its neighboring lower second molar (37) with constant root length, also radiographically by means of two panoramic radiographs (OPGs) produced 12 days and 20 months postoperatively. A method for calculation of the root length as a multiple of the crown length is introduced, using accurately reproducible landmarks defining the coronal and apical endpoints of the examined tooth in all OPGs of the series. This method minimizes error due to different magnifications within the set of radiographs. RESULTS: Using the method introduced, the root length of the constant tooth 37 varied at 2.7% within the set of OPGs, whereas it revealed a 5.6% variation when the evaluation was based on direct measurement. Based on the described method, T revealed a postoperative root growth of approximately one-third of its final length and showed clinically no pathological findings during the observation period. CONCLUSIONS: Our results indicate that with the described method root growth assessment on panoramic radiographs is more accurate than with direct measurement.

A method to calculate angular disparities between object and receptor in "paralleling technique".

OBJECTIVE: To develop an image analysis method for calculation of angular disparities between an object, temporarily equipped with a reference system, and a radiographic receptor. MATERIALS AND METHODS: A mathematical method based on a reference system containing two metallic spheres is developed, allowing calculation of inclination between the inter-spherical axis and the digital image receptor using image features. Experimental evaluation was done in standardized projection geometry for two sphere sizes at four randomly chosen inclinations per size, with each radiograph assessed three times. Truth was assessed threefold from photographs obtained at each inclination. RESULTS: Mean standard deviation between single assessments was 2.6 degrees. Significant differences (P Maloney/Rastogy=0.00) were found between absolute values of truth and calculated values (mean: -0.9 degrees; range: -6.0 degrees; 3.6 degrees), indicating a significant lack of accuracy. CONCLUSIONS: Although so far not sufficiently accurate, the method yields information relevant for correction of distortion in intra-oral radiology.